A gunman opened fire at a TSA document checkpoint, killing a TSA officer, then continued firing rounds inside the terminal

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Paul Ciancia (inset) is suspected of killing a TSA officer and wounding two other people at a security checkpoint at Los Angeles International Airport on Friday.

The FBI announced Saturday that federal prosecutors will file murder charges against the man accused of using an assault rifle to shoot his way through security at Los Angeles International Airport, killing a Transportation Security Administration officer before he was shot and wounded by officers.

Officials said during a press conference that the suspected gunman, if convicted of first degree murder of a federal officer, could face life in prison without parole or the death penalty.

Investigators were still searching for clues into what prompted him to violently descend on the airport.

Police blame Friday's rampage on suspect Paul Anthony Ciancia, a 23-year-old New Jersey native, who opened fire at a TSA document checkpoint of Terminal 3 of the Bradley International Terminal in an incident that also wounded four other people.

Some reports indicated the gunman was inside of the terminal when he began firing, but FBI officials on Saturday clarified he was in the prescreening area.

The rampage began after 9:20 a.m. Friday when the suspect entered the airport with a Smith & Wesson M&P-15 assault rifle and up to 100 rounds of ammo, according to a federal criminal complaint. He took the rifle out of his bag and fired multiple rounds at point-blank range at a TSA officer who was on duty and in uniform, wounding the officer, officials said.

The suspect walked up an escalator, looked back at the officer, and returned to shoot the officer again, killing the officer, the complaint states.

Ciancia was in critical condition at a hospital Saturday.

"He is unresponsive, and we are unable to interview him," FBI officials said Saturday.

Witness Describes Shooting Scene InsideTerminal

A passenger waiting for a flight describes a shooting at LAX. (Published Friday, Nov. 1, 2013)

FBI investigators, meanwhile, were sifting through evidence in the Terminal that remained closed Saturday. Authorities were examining the suspected gunman's path through the airport, how he allegedly pulled the rampage off, and collected details about his background.

The alleged gunman texted suicidal thoughts before the attack, prompting family members to have officers check on his welfare at his apartment in California, NBC News reported. The suspect possibly had been living in the San Fernando Valley community of Sun Valley.

Witness Recalls LAX Shooting Chaos

Carol Stubbs was on her way to a wedding when a gunman fired a rifle inside a terminal at LAX. Toni Guinyard reports for the NBC4 News at Noon on Friday, Nov. 1, 2013. (Published Friday, Nov. 1, 2013)

Officials found a handwritten note by the suspect that detailed his anger and malice toward the TSA. In the letter, he allegedly wrote of his conscious decision to kill multiple TSA employees, FBI officials said.

The letter stated that Ciancia wanted to "instill fear in the traitorous minds" of TSA agents.

Passengers Talk of Confusion in LAX Shooting

In addition to the weapon, agents found anti-government material critical of the federal government and the TSA in his gun bag. The material included literature outlining an alleged conspiracy to create a single global government, sources told NBC News.

Also discovered at the scene were five magazine clips of ammunitions for the rifle. It was unknown how he got the weapon.

FBI agents armed with a warrant searched Ciancia’s home in the LA area late Friday, FBI spokeswoman Laura Eimiller told Reuters.

One witness said the alleged shooter approached him with a one-word question.

"All he said was, 'TSA?' Just like that," Leon Saryan told MSNBC.

The shooting prompted evacuations and halted flights. All terminals were re-opened on Saturday.

Officials credited a quick response to the threat with possibly preventing more casualties.

"There were more than 100 more rounds that could've literally killed everybody in that terminal today," Mayor Garcetti said during a Friday news conference, lauding law enforcement's response that he said saved many lives.

One of the officers who shot the suspected gunman was identified as Sgt. Steve Zouzounis with the LAX Police Department. Four other LAX officers were also involved in the confrontation.

The incident prompted security reviews, however, both the TSA and Homeland Security officials said the shooting would not lead to arming TSA officers as airport security is in the hands of airport police and other city and county law enforcement officials, NBC News reported.

TSA officers are not sworn members of law enforcement, and they are not trained to use firearms, officials said.

The victims, meanwhile, continue to recover. Firefighters treated six people in the aftermath of the shooting, taking five of them to hospitals.

Killed was 39-year-old TSA Officer Gerardo I. Hernandez, a father of two. A second TSA agent was wounded.

Another victim was in fair condition. A third person was treated and released.

LAPD Chief Charlie Beck asked officers to place black bands on their badges to honor Hernandez, until he is laid to rest.

Officials were also honoring the TSA officer, the first such agent to die in the line of duty, by lighting up in blue the iconic LAX pylons that stand at the entrance to the airport.